1. Field of the Invention
This invention relates to quantum computing and, more specifically, to solid state quantum computing qubits with superconducting materials.
2. Discussion of Related Art
Research on what is now called quantum computing traces back to Richard Feynman, See, e.g., R. Feynman, Int. J. Theor. Phys., 21, 467-488 (1982). Feynman noted that quantum systems are inherently difficult to simulate with conventional computers but that observing the evolution of a quantum system could provide a much faster way to solve some computational problems. In particular, solving a theory for the behavior of a quantum system commonly involves solving a differential equation related to the Hamiltonian of the quantum system. Observing the behavior of the quantum system provides information regarding the solutions to the equation.
Further efforts in quantum computing were initially concentrated on “software development” or building of the formal theory of quantum computing. Software development for quantum computing involves attempting to set the Hamiltonian of a quantum system to correspond to a problem requiring solution. Milestones in these efforts were the discoveries of the Shor and Grover algorithms. See, e.g., P. Shor, SIAM J. of Comput., 26:5, 1484-1509 (1997); L. Grover, Proc. 28th STOC, 212-219 (1996); and A. Kitaev, LANL preprint quant-ph/9511026 (1995). In particular, the Shor algorithm permits a quantum computer to factorize natural numbers. Showing that fault-tolerant quantum computation is theoretically possible opened the way for attempts at practical realizations of quantum computers. See, e.g., E. Knill, R. Laflamme, and W. Zurek, Science, 279, p. 342 (1998).
One proposed application of a quantum computer is the factoring of large numbers. In such an application, a quantum computer could render obsolete all existing encryption schemes that use the “public key” method. In another application, quantum computers (or even a smaller scale device such as a quantum repeater) could enable absolutely safe communication channels where a message, in principle, cannot be intercepted without being destroyed in the process. See, e.g., H. J. Briegel et al., LANL preprint quant-ph/9803056 (1998) and the references therein.
Quantum computing generally involves initializing the states of N qubits (quantum bits), creating controlled entanglements among the N qubits, allowing the quantum states of the qubit quantum system to evolve under the influence of the entanglements, and reading the qubits after they have evolved. A qubit quantum system is conventionally a system having two degenerate quantum states, where the state of the qubit quantum system can have non-zero probability of being found in either degenerate state. Thus, N qubit quantum systems can define an initial state that is a combination of 2N states. The entanglements between qubits and the interactions between the qubits and external influences control the evolution of the distinguishable quantum states and define calculations that the evolution of the quantum states perform. This evolution, in effect, can perform 2N simultaneous calculations. Reading the qubits after evolution is complete determines the states of the qubit quantum systems and the results of the calculations.
Several physical systems have been proposed for the qubits in a quantum computer. One system uses chemicals having degenerate nuclear spin states, see U.S. Pat. No. 5,917,322, “Method and Apparatus for Quantum Information Processing”, to N. Gershenfeld and I. Chuang. Nuclear magnetic resonance (NMR) techniques can read the spin states. These systems have successfully implemented a search algorithm, see, e.g., J. A. Jones, M. Mosca, and R. H. Hansen “Implementation of a Quantum Search Algorithm on a Quantum Computer,” Nature, 393, 344-346 (1998) and the references therein, and a number ordering algorithm, see, e.g., Lieven M. K. Vandersypen, Matthias Steffen, Gregory Breyta, Costantino S. Yannoni, Richard Cleve and Isaac L. Chuang, “Experimental Realization of Order-Finding with a Quantum Computer,” LANL preprint quant-ph/0007017 (2000), Phys. Rev. Lett., Vol. 85, No. 25, 5452-55 (2000) and the references therein. The number ordering algorithm is related to the quantum Fourier transform, an essential element of both Shor's algorithm for factoring of a natural number and Grover's Search Algorithm for searching unsorted databases, see T. F. Havel, S. S. Somaroo, C.-H. Tseng, and D. G. Cory, “Principles and Demonstrations of Quantum Information Processing by NMR Spectroscopy, 2000,” LANL preprint quant-ph/9812086 V2 (1999), and the references therein. However, efforts to expand such systems to a commercially useful number of qubits face difficult challenges.
Another physical system for implementing a qubit includes a superconducting reservoir, a superconducting island, and a dirty Josephson junction that can transmit a Cooper pair (of electrons) from the reservoir into the island. The island has two degenerate states. One state is electrically neutral, but the other state has an extra Cooper pair on the island. A problem with this system is that the charge of the island in the state having the extra Cooper pair causes long range electric interactions that interfere with the coherence of the state of the qubit. The electric interactions can force the island into a state that definitely has or lacks an extra Cooper pair. Accordingly, the electric interactions can end the evolution of the state before calculations are complete or qubits are read. This phenomenon is commonly referred to as collapsing the wavefunction, loss of coherence, or decoherence. See Y. Nakamura, Yu. A. Pashkin and J. S. Tsai “Coherent Control of Macroscopic Quantum States in a Single-Cooper-Pair Box,” Nature V. 398 No. 6730, P. 786-788 (1999), and the references therein.
Another physical system for implementing a qubit includes a radio frequency superconducting quantum interference device (RF-SQUID). See J. E. Mooij, T. P. Orlando, L. Levitov, Lin Tian, Caspar H. van der Wal, and Seth Lloyd, “Josephson Persistent-Current Qubit,” Science 285, 1036-39 (Aug. 13, 1999), and the references therein. The energy levels of the RF-SQUID correspond to differing amounts of magnetic flux threading the SQUID ring. Application of a static magnetic field normal to the SQUID ring may bring two of these energy levels, corresponding to different magnetic fluxes threading the ring, into resonance. Typically, external AC magnetic fields are also applied to pump the system into excited states so as to maximize the tunneling frequency between qubit basis states. A problem with this system is that the basis states used are not naturally degenerate and the required biasing field has to be extremely precise. This biasing is possible for one qubit, but with several qubits, this bias field fine-tuning becomes extremely difficult. Another problem is that the basis states used are typically not the ground states of the system, but higher energy states populated by external pumping. This requires the addition of an AC field generating device, whose frequency will differ for each qubit as the individual qubit parameters vary.
The race to create the first scalable, practical, and powerful solid state quantum computer has existed for over ten years. Ever since the notion of a quantum computer first became evident with Feynman in 1982, scientists have been creating qubits of various forms. There are currently a number of disclosed qubits, where the quantum states are realized in the doubly degenerate ground states of the flux in a superconducting loop. Inevitably, these qubits are only useful when controlled by magnetic fields, or by some other means which couple the qubit to the environment or provide other potential sources of decoherence. In order to overcome these sources of decoherence, a large amount of overhead is required to control and harvest the quantum power available from the qubit. However, the means by which this can be accomplished has as yet eluded scientists. Thus, there is a need for a qubit which does not require the coupling magnetic fields, but which can be controlled by applying and reading currents and voltages.
There therefore exists a need for integrated solid state structures that can form the basic building blocks out of which integrated circuits using quantum effects can be built. The desired structures are such that they can be read from, written to and operated on in an efficient and scalable manner.